1. Field of the Invention
The present invention relates to a digital broadcast reception apparatus which receives a digital broadcast, and particularly to a digital broadcast reception apparatus which can carry out a channel selection and setting for each area.
2. Description of the Related Art
In recent years, along with the development of broadcast communication technology and code compression technology, each kind of broadcast has also become increasingly digitalized, and a broadcasting technology has shifted from analog broadcasting to digital broadcasting. In accordance with digitalization in a broadcast sector, in a broadcast reception apparatus which receives a broadcast too, a specification thereof has been changed to a specification for receiving the digital broadcasting, and a digital broadcast reception apparatus is presently being provided which meets a standard by which the digital broadcast is transmitted in a relevant country or area.
In this kind of digital broadcast reception apparatus, in the event that received power of a received broadcast signal is of an appropriate value or smaller, the received broadcast signal is amplified into a signal with a favorable S/N ratio. As opposed to this, in the event that the received power of the received broadcast signal is large beyond a tolerance level, as the received broadcast signal cannot be demodulated due to a distortion occurring therein, the received broadcast signal is attenuated into a condition in which it is receivable. As the received power of the broadcast signal varies depending on a channel frequency or a reception environment of the broadcast signal, it is necessary to set an amount of amplification or attenuation in accordance with the received power thereof.
For this reason, in the digital broadcast reception apparatus, in order to set the amount of amplification or attenuation of the broadcast signal so as to make the received power of the broadcast signal in an antenna most appropriate, a VGA (Variable Gain Amplifier) is provided which carries out an AGC (Automatic Gain Control) over a radio frequency signal received in the antenna. Furthermore, as a digital broadcast reception apparatus which adjusts the amount of amplification or attenuation of the broadcast signal by means of the VGA, one is provided which perceives a reception condition, using a demodulated signal, and sets a gain amount in the VGA, based on the reception condition (refer to JP-A-2006-50585).
Also, as described in JP-A-2006-50585 too, some digital broadcast reception apparatus include an automatic presetting function which searches for and sets a channel which can be received in an installation area. When the receivable channel is searched for using the automatic presetting function, received frequencies are switched in order and, when a tunable received frequency is found, the received frequency found is set and stored. By this means, it is possible to automatically set the channel which can be received in the area in which the digital broadcast reception apparatus is installed.
Meanwhile, the received power of the broadcast signal varies depending on the channel frequency or reception environment of the broadcast signal and, in the analog broadcasting, even in the event that the received power thereof is large, it is possible to carry out a demodulation and decoding of the broadcast signal but, in the digital broadcasting, in the event that the received power is large beyond the tolerance level, it is not possible to carry out the demodulation or the decoding. That is, as a modulation method used in the digital broadcasting, a digital modulation method is used, such as a PSK (Phase Shift Keying), an 8-VSB (Vestigial Side Band) or a QAM (Quadrature Amplitude Modulation), but, an error occurs when the demodulation is carried out by means of the demodulation method, causing a reception abnormality. For this reason, in the digital broadcast reception apparatus, it is necessary, by attenuating the gain of the broadcast signal input through the antenna, as described heretofore, to place the broadcast signal in a condition in which the demodulation and decoding thereof are possible.
However, in a case in which a channel setting using the automatic presetting function is carried out, it is often carried out without varying the gain of the received broadcast signal. For this reason, regarding a digital broadcast on a channel on which the received power of the received broadcast signal is large, a normal reception cannot be found due to an error occurring at a time of the demodulation, and the channel setting becomes impossible. In particular, as a digital broadcast signal with a lower channel frequency has larger received power when it is input through the antenna, there is a case in which the channel setting cannot be carried out on a channel using an area in which a frequency in a VHF band is low.
Also, in the digital broadcast reception apparatus of JP-A-2006-50585, an arrangement is such that, by finding a signal error after the demodulation and decoding, the reception condition is recognized, and the gain of the broadcast signal input through the antenna is changed. However, in the event that a digital broadcast on a channel, on which the received power of the broadcast signal is large beyond the tolerance level, has been received, it is impossible to demodulate and decode the received broadcast signal. For this reason, in the digital broadcast reception apparatus of JP-A-2006-50585 too, when the channel setting using the automatic presetting function is carried out, there is a case in which it is impossible to find the digital broadcast on the channel with the large received power, and carry out the channel setting.